In a system that reforms the fuel supplied to an engine using engine exhaust heat and generates, in part, hydrogen and the like, the fuel is reformed through an endothermic reaction using the engine exhaust heat, so that an improve in efficiency can be expected by recovering the exhaust heat. When hydrocarbon fuel, such as gasoline, is reformed so as to supply a reformed gas containing hydrogen to an engine, an increase in efficiency of the engine can be expected because a decrease in the pumping loss, an improvement in the combustion efficiency, and an increase in the combustion speed can be achieved. Moreover, dehydrogenation reaction is performed from hydrocarbon fuel, such as gasoline, so that the remaining hydrocarbon fuel after generating hydrogen will have a high octane number. This increases the knocking resistance, so that an improvement in the heat efficiency can be achieved by increasing the compression ratio. However, when the reformer is mounted in an exhaust pipe of an engine, the temperature of the reformer fluctuates depending on the operation conditions of the engine, and therefore the reformed amount fluctuates depending on the operation conditions. Thus, the amount of hydrogen and the amount of high-octane fuel after reforming will fluctuate. By performing the optimum engine control in response to these fluctuations, the heat efficiency can be increased.
As the conventional engine system with a reformer, for example, as described in Patent Literature 1, there is a technique, wherein a sensor is provided in the reformed gas and whereby the hydrogen concentration of the reformed gas is detected.